1. Field of the Invention
The invention relates to a production method for a fuel cell. More particularly, the invention relates to a production method for a fuel cell, which makes it possible to produce a fuel cell with reduced contact resistance.
2. Description of the Related Art
Among various types of fuel cells, great attention has been given to a polymer electrolyte fuel cell (hereinafter referred to as a “PEFC”), as the optimum power source for an electric vehicle. This is because the PEFC can be operated at a rather low temperature, and exhibits high energy conversion efficiency of 50% to 60%. Also, a start-up time of the PEFC is short, and a system of the PEFC is compact and light-weight. For an electrolyte of the PEFC, a cation-exchange resin membrane is used. The cation-exchange resin membrane is used as a cation conductive membrane. The cation conductive membrane of the PEFC has a proton (hydrogen ion (H+)) exchange group in a molecule. When being moisturized to saturation, the cation conductive membrane exhibits specific resistance of equal to or lower than 20 Ωcm, and serves as a proton conductive electrolyte. The saturated moisture content of the electrolyte membrane reversibly changes based on a temperature. Namely, when the PEFC is being operated, the electrolyte membrane is kept saturated with moisturizing water that is supplied, in the form of water vapor, to fuel gas and oxidizing gas in order to prevent a moisture loss from the electrolyte membrane, and with water that is generated by an electrochemical reaction which occurs on the cathode side.
The PEFC mainly includes an electrolyte membrane, a fuel electrode (hereinafter, referred to as an “anode”), an air electrode (hereinafter, referred to as a “cathode”), and separators. Such a PEFC is usually formed by the following method. First, the cathode is joined to one of both surfaces of the electrolyte membrane, which is formed to be a thin-film in advance, and the anode is joined to the other surface, whereby a membrane electrode assembly (hereinafter, referred to as a “MEA”) is formed. Then, a diffusion layer is joined to each of both surfaces of the MEA, whereby a membrane-electrode-diffusion layer assembly (hereinafter, referred to as a “MEGA”) is formed, and a separator is then fitted to the MEGA on each of the anode side and the cathode side, whereby the PEFC as a module is produced.
However, the PEFC produced by this method has the following problems. When the cathode and the anode are joined to the surfaces of the thin-film electrolyte membrane, the porosity of the cathode and the anode tends to be reduced. Also, the electric power generation efficiency of the PEFC tends to be reduced due to contact resistance which occurs, for example, if the anode and the cathode are not sufficiently joined to the thin-film electrolyte membrane.
Accordingly, technologies for reducing contact resistance in a fuel cell have been proposed. For example, Japanese Patent Application Publication No. JP (A) 06-176771 discloses the following technology. In this technology, the minimum amount of ion-exchange resin required for joining an ion-exchange membrane and an electrode to each other is moisturized and joined to an electrode surface by forming an ion-conductive thin-film, whose glass-transition temperature is lower than that of the ion-exchange membrane, on one of or both of the surfaces of the ion-exchange membrane, whereby contact resistance is reduced.
However, the technology disclosed in Japanese Patent Application Publication No. JP (A) 06-176771 has the following problem. Although the contact resistance can be reduced, the production method for a fuel cell according to this technology becomes complicated, since multiple ion-exchange membranes need to be provided.
A PEFC module is usually produced by joining/fitting components such as an anode including a catalytic layer and a cathode including a catalytic layer and separators to an electrolyte membrane which has been formed to be a thin-film in advance. However, the electrolyte membrane of the PEFC produced by this method has problems that yield thereof is low, and/or that durability thereof is low. The PEFC produced by this method has another problem that relatively high contact resistance occurs in the fuel cell, and therefore the electric power generation efficiency tends to be reduced.
When the PEFC is produced by the above-mentioned conventional method, the catalytic layer of the anode and the catalytic layer of the cathode are joined to the thin-film electrolyte membrane. Examples of a typical method for joining the electrolyte membrane and the catalytic layers to each other include a method in which the catalytic layers are transcribed to the electrolyte membrane (hereinafter, this method will be referred to as a “transcription method”), and a method in which the catalytic layers are sprayed on the electrolyte membrane (hereinafter, this method will be referred to as a “spray coating method”). However, the transcription method has the following problem. When the catalytic layers are joined to the electrolyte membrane by the transcription method, the catalytic layers needs to be pressed to the thin-film electrolyte membrane. Accordingly, the porosity of the catalytic layers is reduced during pressing. Also, the spray coating method has the following problem. When the catalytic layers are joined to the electrolyte membrane by the spray coating method, although it is unlikely that porosity of the catalytic layers is reduced, the electrolyte may be damaged since a solvent is used during the spray coating.